Fix some readme and script bugs in examples (take 2).

This can go either way. Adding users to the "docker" group is convenient, but makes them essentially root-equivalent so it's not for especially security-conscious users.

Discovering this state shouldn't be too hard. Try 'docker ps'. If it works you win. If it fails, _and_ $DOCKER_HOST is unset or starts with unix://, then try again with sudo.